Raspbian Jessie w/ external / on btrfs ?

Quite successfully managed to relocate / to an external disk.  In my case, the external disk is an old 1TB hard drive in one of these snazzy docks (does not pass S.M.A.R.T. data).  Then I tried it with an SSD to see if it was any faster.  It was, but not enough to justify its immigration from my desktop as a cache device.

That’s all well and good, but I’d like to achieve some redundancy.

First, I started with MDADM, that didn’t work.  Then I moved to btrfs.  Seemed promising, but I get a kernel panic during boot.

Guides used:

Will do more fiddling later..